This invention relates to a process for electrolytic deposition of a metal layer on zirconium or a zirconium alloy, and more particularly to an improved process for electrolytic deposition of a metal film such as a copper film on the internal surface of a long hollow cylindrical tube of zirconium or a zirconium alloy.
Nuclear reactors are presently being designed, constructed and operated with the nuclear fuel being contained in fuel elements which can have various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a corrosion-resistant, non-reactive, heat conductive container or cladding. The elements are assembled together in a lattice at fixed distances from each other in a coolant flow channel or region forming a fuel assembly, and sufficient fuel assemblies are combined to form the nuclear fission chain reacting assembly or reactor core capable of a self-sustained fission reaction. The core in turn is enclosed within a reactor vessel through which a coolant is passed.
The cladding serves several purposes and two primary purposes are: first, to prevent contact and chemical reactions between the nuclear fuel and the coolant or the moderator if a moderator is present, or both if both coolant and moderator are present; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant or the moderator, or both if both coolant and moderator are present. Common cladding materials are zirconium and its alloys as well as others that are commonly used. The failure of the cladding, i.e., a loss of the leak tightness, can contaminate the coolant or moderator and the associated systems with radioactive long-lived products to a degree which interferes with plant operation.
Problems have been encountered in the operation of nuclear fuel elements which employ certain metals and alloys as the clad material due to mechanical or chemical reactions of these cladding materials under certain circumstances. Zirconium and its alloys, under normal circumstances, are excellent nuclear fuel claddings since they have low neutron absorption cross sections and at temperatures below about 750.degree. F. (about 398.degree. C.) are strong, ductile, extremely stable and non-reactive in the presence of demineralized water or steam which are commonly used as reactor coolants and moderators.
However, fuel element performance has revealed a problem with the brittle splitting of the cladding due to the combined interactions between the nuclear fuel, the cladding and the fission products produced during nuclear fission reactions. It has been discovered that this brittle splitting is due to localized mechanical stresses resulting from the differential expansion of the fuel into contact with the cladding (i.e., stresses in the cladding are localized at cracks in the nuclear fuel). Corrosive fission products are released from the nuclear fuel and are present at the intersection of the fuel cracks with the cladding surface. Fission products are created in the nuclear fuel during the fission chain reaction occurring during operation of a nuclear reactor. The localized stress is exaggerated by high friction between the fuel and the cladding.
A composite cladding container disclosed in U.S. patent application Ser. No. 522,769, now abandoned in favor of continuation application Ser. No. 725,824, filed Sept. 23, 1976, has improved performance and resistance to mechanical and chemical reactions. This application was filed in the names of Gerald M. Gordon and Robert L. Cowan on Nov. 11, 1974 and is assigned to the same assignee as the present invention. The composite cladding container is comprised of an outer layer consisting of zirconium or a zirconium alloy that has bonded on the inside surface of the outer layer a protective layer of a material selected from the group consisting of copper, nickel, iron or alloys thereof. Various methods are disclosed for coating the inside surface of the outer layer of zirconium or a zirconium alloy with the protective layer, and one of the methods involves electroplating. Copper is a particularly preferred material for use as the protective layer.
A novel aqueous electrolytic activating solution and a method for electroplating a metal layer on zirconium and zirconium alloys are disclosed in U.S. Pat. No. 4,017,368, issued Apr. 12, 1977. This application was filed in the names of Daniel E. Wax and Robert L. Cowan on Nov. 11, 1974 and is assigned to the same assignee as the present invention. The electroplating method of this invention is particularly suitable for coating the inside surface of zirconium or a zirconium alloy with the protective layer of copper, nickel or iron (as called for in application Ser. No. 522,769). The first step of the process comprises activating the zirconium or zirconium alloy in an aged aqueous activating solution comprising from about 10 to about 20 grams per liter of ammonium bifluoride and from 0.75 to about 2 grams per liter of sulfuric acid. The solution is aged by immersion of pickled zirconium in the solution for about 10 minutes. The second step of the process comprises electroplating the zirconium material in a plating bath of the metal to be plated on the zirconium material in the presence of an electrode.
An electroplating process requires the use of an electrode approximately the same length as the piece being plated. The nuclear fuel cladding is a tube of about 14 feet in length and about 0.50 inch in internal diameter. This means that an electrode of about 14 feet in length with about 0.125 inch in diameter is required for an electroplating process.
Zirconium materials activated in an ammonium bifluoride-sulfuric acid solution have on the surface a first layer of a black color that is highly adherent to the zirconium substrate and is electrically conductive. This layer is believed to make it possible to initiate electroplating of the zirconium material. These activated zirconium materials also have a second layer on the first layer that is a loosely adhering layer of similar color to the first layer. The presence of this second layer is believed to have an adverse effect on adhesion and to give rise to the possibility of blistering of the coating.
Therefore it is desirable to develop an electrolytic process for plating zirconium materials that includes a step of removing the loosely adhering layer on the zirconium material resulting from the activation of the zirconium material in the ammonium bifluoride-sulfuric acid solution.